Structure of the Roberts Mountains allochthon in the Three Bar Ranch Quadrangle, Roberts Mountains, Eureka County, Nevada

<p> This study focuses on the western Gale Hills located in the western portion of the Lake Mead domain in southern Nevada. The western Gale Hills preserve a record of the Miocene sedimentation and deformation related to the breakup of the hanging walls of the South Virgin-White Hills detachment fault and the Lime Ridge oblique, strike-slip fault of the Lake Mead fault system, the initiation of the right-lateral Las Vegas Valley shear zone in the western Lake Mead domain, and subsequent middle to late Miocene deformation. This study focuses on the lower Horse Spring Formation north of the Las Vegas Valley shear zone. To better understand the stratigraphy and deformation, a detailed geologic map (1:10,000 scale) was produced, data from primary and secondary structures were collected, and ash-fall tuff deposits were dated and correlated through ⁴⁰Ar/³⁹Ar geochronology and tephrachronology. The stratigraphy of the Gale Hills records the initial buttressing of the lower Thumb Member of the Horse Spring Formation onto pre-Tertiary topography. Deposition of the lower Thumb Member records a rapid transgression of the basin margin to the north and northwest across the majority of the Gale Hills. This time was period was then followed by a coarsening up interval and progradation of large alluvial fans in the middle to upper Thumb Member. A transition to a marginal clastic lake in the uppermost Thumb Member then abruptly changed to the Bitter Ridge Limestone algal lake.</p><p> Many studies have evaluated the exhumation history of the Gold Butte block in the eastern Lake Mead domain, which forms the footwall of the major South Virgin-White Hills detachment fault and the relationship with the Frenchman Mountain block. This study shows that the Frenchman Mountain block was just south of the Gale Hills during the early to peak stages of detachment faulting from ca. 17-14 Ma. Two new ⁴⁰Ar/³⁹Ar dates (15.35 Ma) from a prominent ash-fall tuff in the Thumb Member in the northern and southern regions of the western Gale Hills is also in the Frenchman Mountain block. In addition, new tephrachronology correlations have tied Proterozoic-clast debris flows in the western Gale Hills to Proterozoic-clast megabreccia deposits in the Frenchman Mountain block, indicating that the two areas were one connected basin during upper Thumb Member time.</p><p> This study suggests that the left-lateral Government Wash and Southern Gale Hills faults are reactivated northeast-striking, west-down normal faults that were in the correct orientation to be major Riedel prime shears (R') to the right-lateral Las Vegas Valley shear zone. Map and facies relationships show that the Thumb Member deposits were faulted locally during deposition at ca. 15.5 Ma, with increased fault activity and sedimentation rates throughout the Thumb Member after 15.35 Ma and before &sim;14.5 Ma.</p><p> An analysis of structures in the western Gale Hills results in a new model of progressive clockwise rotation and faulting along the Las Vegas Valley shear zone that for the first time honors paleomagnetic results and accounts for all major faulting north of the shear zone. The model is primarily based on clockwise vertical-axis block rotation of domains between oblique left-lateral faults that curve progressively toward the Las Vegas Valley shear zone and terminate into major folds or areas of complex deformation. This model predicts that the western Gale Hills began as a north-northeast elongate block that was reduced in length and elongated in an east-west direction from about 20 to 14 km during translation and rotations. Most of this complex faulting occurred from ca. 13.8 to 8 Ma, after deposition of the Bitter Ridge Limestone.</p>

Thesis--University of Michigan. / From the Bulletin of the Geological Society of America, v. 59, 1948.

There has been confirmation in the last two decades, through instrumental measurements onboard satellites, that the 'solar constant' does, as has long been hypothesized, vary. While there is no consensus as to the best method for estimating past variations in solar output, it seems likely that over the last 500 years, the sun has played a role in the changing climate. However, there is little evidence to suggest that changes in irradiance are having a large impact on the current warming trend. A complementary approach of empirical and model-based analysis is used to determine if the climate effects of an estimated change in solar irradiance were significant in the pre-industrial era and what climate patterns emerge in response to reduced solar forcing at that time. Also investigated is the modification of solar-induced climate patterns by a hitherto underrepresented forcing - changes in Earth's orbit - and how solar and orbital forcing compare to that of increasing atmospheric greenhouse gas concentration. Finally a brief analysis of the effects of a Maunder Minimum-like solar irradiance on a climate forced by doubled CO2 is undertaken. Clear evidence is established herein for the ability of relatively small changes in solar irradiance to impact the global climate. Both at the century scale and at shorter periodicities, the variability of the solar 'constant' is influential in defining the global mean climate and more importantly, the regional characterization of that climate. Influencing the preferred mode of decadal variability, such as North Atlantic Oscillation, solar variability alters the mean climate for northern Europe and the North Atlantic region. The global response for temperature is found to be near-linear, while precipitation is more complex. Excitation of important feedbacks, such as sea-ice, plays an important role in determining the resulting pattern of response and ensures that even a much smaller forcing (solar variability) can exert a similar fingerprint to that of a larger forcing (greenhouse gases). Orbital forcing, typically excluded from model experiments for the decadal-to-centennial scale, is found to provide important modification of regional response and may be critical for determining a more accurate 'forecast' for future climate.

This study was designed with the overriding goal of improving our understanding of the nature, causes, and impacts of past climatic conditions in the High Arctic and to evaluate the potential impacts of future climatic warming. Specifically, the focus of this project was centered on Upper and Lower Murray Lakes (81° 21' N, 69° 32' W) on northern Ellesmere Island, Nunavut, Canada. Sediment cores were collected from each of the lakes in order to reconstruct past climate and environmental variability and space-borne synthetic aperture radar (SAR) data were used to evaluate recent variability in the ice cover of these lakes. The climatic setting and physical characteristics of Lower Murray Lake has led to the formation and preservation of annually laminated sediments (varves). Varve deposition began ca. 5200 calendar years BP and continued through 2004 AD, providing an annual record of sediment accumulation spanning the past 5200+ years. Annual mass accumulation was correlated to regional July temperatures providing a means of quantitatively evaluating past temperature changes in the region. The temperature reconstruction suggests that recent temperatures are ∼2.6°C higher than minimum temperatures observed during the Little Ice Age, maximum temperatures during the past 5200 years exceeded modern values by ∼0.6°C, and that minimum temperatures observed approximately 2900 varve years BC were ∼3.5°C colder than recent conditions. SAR observations of the ice cover Upper and Lower Murray Lake were used to assess the potential effects of past and future temperatures on lake-ice conditions. Under current climatic conditions the lakes average several weeks of ice-free conditions in August and early September, although in some years a continuous ice cover persists throughout the year. The relationship between summer temperature and ice melt at the lakes suggests that recent warming in the High Arctic has forced the lakes across a threshold from a state of perennial ice cover to seasonal melting. Projected future warming will significantly increase the duration of ice free conditions on Upper and Lower Murray Lakes. Ice-out is predicted to occur between 6 and 28 days earlier for every 1°C of warming.

Thesis (M.A.)--University of Hong Kong, 1972. / Mimeographed.

Thèse--Université de Paris. / "Bibliographie": p. [5]-10.

Thesis (M.A.)--University of Hong Kong, 1972. / Also available in electronic format. Also available in print.

Thesis (Ph. D.)--Columbia University, 1939. / Vita. "Reprint of Colby monographs, no. 7, 1938." Bibliography: p. 62-64.

Thesis (Ph. D.)--Columbia University, 1939. / Vita. "Reprint of Colby monographs, no. 7, 1938." Bibliography: p. 62-64.